Findings of decreased brain malate after morphine administration and of elevated brain citrate after antagonist administration have initiated studies of other substrates and of enzymes of the citric acid cycle. Amino acids related to citric acid cycle substrates as well as the GABA shunt will be studied. Evaluation of the energy storage compound, P-creatine, in mice chronically implanted with morphine pellets will be explored by assay of other high energy phosphate compounds, ATP, ADP, and AMP.